Summed time-of-flight mass spectrometry utilizing thresholding to reduce noise

Abstract

A summed TOFMS having a filter for identifying detector outputs that are likely the result of noise rather than ions striking the ion detector. The TOFMS stores a plurality of data values at locations specified by a register that counts clock pulses. The filter receives the ion measurements from the ion detector and generates an output measurement value corresponding to each ion measurement. The filter sets the output measurement value to a predetermined baseline value if the filter determines that the ion measurement is noise, otherwise the filter sets the output measurement value to the ion measurement. An adder, responsive to the clock signal, forms the sum of the data value specified by the register value and the output measurement value and stores the sum in the memory at the location corresponding to the register value.

What is claimed is:  
     
       1. A mass spectrometer comprising: 
       a clock for generating a series of clock pulses;  
       an ion accelerator for generating an ion pulse in response to a start signal;  
       a register for storing a register value that is incremented on each of said clock pulses;  
       an ion detector, spatially separated from said accelerator, for generating an ion measurement indicative of the ions striking said detector during each of said clock pulses;  
       a memory having a plurality of data values at locations specified by said register value;  
       a filter for receiving said ion measurements and generating output measurement values corresponding to each ion measurement, said filter setting said output measurement value to a predetermined baseline value if said filter determines that said ion measurement is noise and said filter setting said output measurement value to said ion measurement otherwise; and  
       an adder, responsive to said clock signal, for forming the sum of said data value specified by said register value and said output measurement value and storing said sum in said memory at said location corresponding to said register value.  
     
     
       2. The mass spectrometer of  claim 1  wherein said filter determines that one of said ion measurements is noise if said ion measurement is within a predetermined threshold value of said baseline. 
     
     
       3. The mass spectrometer of  claim 1  wherein said filter determines that one of said ion measurements is not noise if said measurement is greater than a first threshold and a function of said ion measurements corresponding to a predetermined number of adjacent register values is greater than a second threshold. 
     
     
       4. The mass spectrometer of  claim 3  wherein said function comprises the sum of said ion measurements. 
     
     
       5. The mass spectrometer of  claim 3  wherein said function comprises the number of said ion measurements that is greater than a predetermined number. 
     
     
       6. The mass spectrometer of  claim 3  wherein said function is the value of a finite impulse response filter operating on a sequence of said ion measurements. 
     
     
       7. The mass spectrometer of  claim 6  wherein said finite impulse response filter comprises a function determined by the sequence of ion measurements generated by said ion detector when said ion detector detects a predetermined class of ions.